This invention relates to a method and device for tracer correction in a welding robot.
When welding large structures by a welding robot of the simple play-back system in which the robot merely repeats the line of actions as taught and welding is performed at a low tacking accuracy, it is frequently the case that there is around 5 to 10 mm error between the taught weld line and the weld line of the next work, hence it is difficult to make satisfactory welding.
To compensate for this drawback, it is practiced to cause the robot to follow in on-line tracing the actual weld line by detecting torch tip error against a weld line through the monitoring of welding current or by detecting the torch tip error against the weld line through the detection of arc produced by welding by means of a TV camera installed to the welding torch.
The coordinate system for detecting the error differs according to the type of error detector (hereinafter referred to as a sensor) being employed. For example, as shown in FIG. 1(a), a sensor for monitoring welding current detects the error in the A--A' and B--B' directions which are respectively parallel and orthogonal with respect to a torch 3, while a sensor employing a TV camera detects the torch tip error in the C--C' and D--D' directions which are inclined by 45 degrees with respect to the torch 3 as shown in FIG. 2, thus the coordinate systems for detecting the torch tip error differ one from the other. The coordinate system of FIG. 1 and that of FIG. 2 are hereafter referred to as the U coordinate system and the V coordinate system for the purpose of explanation. In prior art, it has been common to make the robot control axis correspond to the detection coordinate system of a sensor being used to cope with the difference in the detection coordinate systems for each type of the sensor.
However, with robots having such control axis, weldable works are unaviodably limited, and welding narrow parts such as the inside of a box is difficult. A device has been proposed to enable welding of narrow parts in which an arm rotatable in the horizontal and vertical planes as shown in FIG. 3(a) and (b) is provided between a robot body 5 and the torch 3.
It should be noted here that the detection coordinate system of the sensor installed to the welding torch is based on the torch while the torch posture is based on the coordinate system of the robot control axis. As a result, the two coordinate systems differ from each other. Particularly when a robot performs welding while changing the posture of a wrist 4 as shown in FIG. 4, a correcting operation, that is, converting the torch tip error detected by the senser into the robot displacement has become necessary.
In addition, when groove forms vary as shown in FIG. 5(a) through FIG. 5(d), welding conditions such as torch movement vary accordingly, and in many cases a sensor that can be used varies. In such cases, it is preferable to select and install to the torch 3 a sensor optimum to individual weldment. Prior art robot controllers have failed to make the above correcting operation in response to each type of sensor.